Generally, a motor vehicle automatic transmission includes a number of gear elements coupling its input and output shafts, and a related number of torque establishing devices such as clutches and brakes which are selectively engageable to activate certain gear elements for establishing a desired speed ratio between the input and output shafts. The brake can be of the band type or disk type; engineering personnel in the automotive art refer to disc type brakes in transmissions as "clutches" or "reaction clutches". As used herein, the terms "clutches" and "torque transmitting devices" will be used to refer to brakes as well as clutches. The input shaft is connected to the vehicle engine through a fluid coupling, such as a torque converter, and the output shaft is connected directly to the vehicle wheels.
In the type of transmission involved in this invention, the clutches are fluid operated, and each develops torque capacity in relation to the fluid pressure in its apply chamber once such apply chamber has been filled. Shifting from one forward speed ratio to another involves releasing the pressure supplied to an off-going clutch associated with the current speed ratio while initiating the supply of fluid pressure to an on-coming clutch associated with the desired speed ratio. Shifts performed in this manner are termed clutch-to-clutch shifts and require precise timing in order to achieve high quality shifting.
The present invention is directed to upshifting the transmission from a current speed ratio to a desired speed ratio which is numerically lower than the current ratio, the speed ratio being defined as the transmission input speed divided by the transmission output speed. Thus, an upshift involves a pulldown or reduction of the input speed.
The quality of an upshift depends on the cooperative operation of several functions, such as pressure changes and the timing of control events. One of the measures of upshift quality is turbine flare. Turbine flare during an upshift occurs when one or both of the following conditions are satisfied:
a) The on-coming clutch is underfilled when the off-going clutch is released. In this case, the turbine flare starts when the off-going clutch starts to slip, and the amount of such slip is indicative of the degree of flare.
b) The initial on-coming clutch pressure after the fill period is too low. In this case, the on-coming clutch does not have the required torque capacity to hold the turbine speed or pull it down, and the turbine flare starts during the transition from off-going clutch to on-coming clutch.
Typically, an upshift control, whether open loop or closed loop, is designed to manage the smooth transfer of torque from one clutch to the other within a given time period. If the time period expires, full pressure is applied to the on-coming clutch. If the on-coming clutch does not already have a reasonably high pressure, the sudden application of high pressure can have the effect of a shock as a result of a undesirably rapid change of axle torque. Uncontrolled turbine flare leads to such a condition.